As set out in the parent application, microwave sintering is believed to be effective for conversion of loose particulate material which is packed into a small mold. It is especially useful in the manufacture of cast devices which are sufficiently small to fit in the microwave sintering process. Examples of devices which benefit from microwave sintering and which are especially enhanced by such sintering techniques include drill bit inserts. While many examples could be noted, this is an especially important device for microwave sintering. A typical drill bit insert measures about 1/2 inch in diameter and has a length of about 1 inch. At one end, it is normally formed of tungsten carbide particles supported in a softer metal alloy which normally is formed of a number of metals but especially featuring cobalt. At the exposed or cutting end of the insert, the tungsten carbide sintered body is then capped with a diamond particle layer. It is secured in place by a cobalt alloy matrix. Quick heating and cooling is important to the fabrication of this composite structure. Different quantities of cobalt are used to form the tungsten carbide (WC hereinafter) body of the drill bit insert while the diamond crown has a different level of cobalt in it. The crown is normally called a polycrystalline diamond compact or PDC. The PDC capped WC body is later inserted into an opening formed in the body of the drill bit. This is fastened in place in an interference fit, i.e., the hole is smaller than the outside diameter of the cylindrical insert, or brazed in place.
If heated with conventional sintering techniques, the heat is maintained for sufficiently long intervals that grain growth occurs. This damages the structural integrity of the completed product. Worse than that, it provides a less than acceptable cobalt alloy dispersion in the different regions. This is undesirable in all aspects.
Sintering by microwave achieves modification of the grain boundaries and also accomplishes the sintering in such a short time interval that the alloy integrity is unchanged. In fact, the finished product exhibits more desirable characteristics. Sintering, in this particular instance, is directed to the fabrication of loose particulate materials into a solid member having structurally sintered yet different regions. This sintering process reduces or avoids multiple intermediate sintering steps otherwise involved in separate WC and PDC components. It also reduces or eliminates the stress that is involved in attaching the PDC layer to the WC component.
Prior to manufacture, the constituent parts of the drill bit insert are powders. They are loosely packed in a mold at a nominal pressure. They are joined together in the mold either by a slight amount of moisture but preferably with a sacrificial wax. This provides just enough adhesive benefit to hold the particles together. During sintering, the wax is driven off in the form of a combustible gas. If the volume is sufficient, the gas can be combusted for easy disposal after it has been vaporized. However, it is not involved in the heating process itself; rather, it is involved in initially adhering the particles together so that they maintain structural integrity at the time of molding and from molding through sintering. The finished products hold together as a result of the sintering process; the sintered drill bit insert has structural integrity as a result of the hard particles and the metal alloy binder which holds them together.
The amount of sintering can be controlled in making a sintered product by simply turning the microwave source off, first placing the unsintered green molded product in the microwave cavity and then turning it on. The present disclosure sets forth a process which is advanced over that. The microwave generator is turned on and left on. An elongate tube, hollow and having a circular cross-section in the preferred form, extends through the microwave cavity and is able to process a series of individual molded green inserts. They are assembled in individual molds. The molds provide structural definition to the profile and hold the particulate ingredients together in the desired profile and shape. That shape is held during the sintering process. Each mold is preferably identical in size and shape to the others so that they can be serially pushed or dropped by gravity through the tube in the microwave cavity. By controlling the velocity, the dwell time of each mold, and, hence, each insert in the cavity can be controlled. By controlling the velocity and the dwell time, the microwave sintering equipment is then simply switched on and left on so long as individual molds are sequentially put into and taken out of the sintering furnace.
The pathway for an individual mold is thus along a conveyor tube. They are introduced at the same rate and they are removed at the same rate. This enables a consistent dwell time to be obtained for every sintered insert.
In some instances, it will be desirable that the individual molded pieces progress through the conveyor tube by gravity and in other instances, they can be forced through the conveyor tube with a positive feed and indexing mechanism. In some occasions, it is more desirable that the conveyor tube be vertical but it will also operate at an inclined angle or horizontally. Vertical and horizontal embodiments are both illustrated and described below.
The present apparatus is therefore summarized as a microwave sintering oven for multiple small pieces. An example is the molding of a drill bit insert which is made of WC and/or PDC in separate layers and which are sintered together to form a unitary device. They are held together by an alloy (primarily formed of cobalt and other high temperature alloys) and are compacted in a small mold. The individual molds are sequentially placed in a conveyor tube and conveyed through a microwave cavity. Heat is created in them. In one embodiment, a preheater is added to raise the temperature of the green molded piece prior to microwave exposure. This helps change the reflectivity, therefore increasing microwave absorption as will be noted. The conveyor tube is provided with a series of individual molds which hold individual work pieces; they progress through it in sequence and are treated thereby having a fixed dwell time sufficient to obtain full sintering. Different techniques are set forth for feeding including gravity feed, operation of an indexing input device, and pushing the mold pieces with a rod inserted into the conveyor tube.